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Recap VSEPR Theory1. Draw Lewis Structure.2. Count number of electron pairs.3. Determine the arrangement of electron
pairs.4. Use atom positions to name molecular
geometry.
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Three areas of electrons: Trigonal Planar• Remove one arm from the electron pair
arrangement for each lone pair present.
Recap
Figure 10.4 Silberberg
SO3 SO2
3
Four areas of electrons: Tetrahedral
Recap
Figure 10.5 Silberberg
CH4 NH3 H2O
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Electron Pair Arrangements
• Five electron pairs:– Atoms at the corners of a trigonal
bipyramid.– Some electron pairs separated by 120
degrees, other by 90 degrees.– Called trigonal bipyramidal.
Figure 10.2 Silberberg
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Electron Pair Arrangements
• Six electron pairs:– Atoms at the corners of an octahedron.– 90 degrees between electron pairs.– Called octahedral.
Figure 10.2 Silberberg
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Where do lone pairs go?• Five electron pairs:
– If one lone pair present is it in an axial or equatorial position?
– Repulsion: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.
Equitorial
Axial
Structures derived from a trigonal bipyramid
Molecular Geometry
Figure 10.7 Silberberg
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Structures derived from an octahedron
Molecular Geometry
Figure 10.8 Silberberg
All positions are identical
Number of lone electron pairs0 1 2 3
3
trigonal planar bent 4
tetrahedraltrigonal
pyramidal bent 5
trigonal bipyramidal see-saw T-shaped linear
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octahedralsquare
pyramidal square planar T-shaped
Tota
l num
ber o
f ele
ctro
n pa
irsSummary of Molecular Geometry
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Molecular Geometry and Polarity
BF3 ClF3F Cl
F
F
PolarNon-Polar
F B
F
F
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Molecular Geometry and Polarity
SiF4 SF4
PolarNon-Polar
F
SiF F
F
F
SF F
F
• By the end of this lecture, you should:− be able predict the molecular geometry of
molecules and ions containing a central atom
− be able to name the geometry of a given species
− understand the positioning of lone pairs to minimise repulsion
− be able to predict the presence of a permanent dipole moment
− be able to complete the worksheet (if you haven’t already done so…)
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Learning Outcomes:
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Questions to complete for next lecture:
1. Give the geometry of the following polyatomic ions and give approximate bond angles.a) AsF6
-
b) PBr4-
c) SF5-
d) SO42-
e) ICl2-
f) N3-
2. A binary compound contains two different elements. What molecular geometries would result in binary compounds that are polar?
3. What molecular geometries would result in binary compounds that are non-polar?